Pulse Code Modulation (PCM) encoders translate an analog signal into a multi-bit PCM word. The coding process entails quantizing or assigning a sample of the analog signal to the nearest one of a number of discrete signal levels or steps. These steps, connected by risers, extend from an origin over a predetermined range of the analog signal. The origin is located on either a step or a riser depending on the type of coding. In mid-step-biased coding, such as .mu.-law coding, the origin is located on the mid-point of a step. Mid-riser-biased coding, such as the A-law coding used in European transmission systems, locates the origin on a riser. This distinction is of significance to the problem of idle channel noise and crosstalk as will be hereinafter discussed.
Successive approximation or feedback encoders are commonly used for both mid-step and mid-riser biased coding. In such encoders, each binary digit of the PCM word is sequentially determined from a comparison of the sampled analog signal vis-a-vis a reference signal. Typically, the first comparison determines the polarity of the sampled analog signal and is made with the reference signal at zero. Through a series of subsequent comparisons, a full PCM word is generated which corresponds to a particular code step. For a further discussion of successive approximation encoders, see Transmission Systems For Communications, published by WECo Inc., Revised Fourth Edition, 1971, pages 583-585.
Idle channel noise and crosstalk are a problem in systems where an analog signal is quantized. The problem is most acute when the encoder is biased, by an accumulation of dc voltage, at or near a code step boundary. Under this condition, a small signal perturbation, such as idle channel noise and crosstalk, is encoded as two code words. This results in enhancement of the original signal perturbation on decoding. In systems which transmit voice signals, this enhancement produces an undesirable audible sound. In mid-step-biased encoders, compensation for the dc voltage is introduced to bias the encoder at the origin. As this position is mid-way between code step boundaries, the likelihood of idle channel noise and crosstalk enhancement is minimized. Utilizing this technique for mid-riser biased encoders, however, biases the coder at a code step boundary and maximizes the likelihood of idle channel noise and crosstalk enhancement. Consequently, the problem of idle channel noise and crosstalk in mid-riser biased encoders is a continuing problem.